The scientists who altered a deadly flu virus to make it more contagious have agreed to suspend their research for 60 days to give other researchers around the world time to discuss the work and determine the best way to proceed. A letter explaining the decision is being published in two scientific journals, Science and Nature, which also plan to publish reports on the research, but in a redacted form omitting details that would let other researchers copy the experiments. The letter is signed by the scientists who produced the new, more contagious form of the flu virus, as well as by other leading flu researchers. The scientists say their work has important public health benefits, but they acknowledge that it has sparked intense public fears that the deadly virus could accidentally leak out of a laboratory, or be stolen by terrorists, and result in a devastating pandemic. A national biosecurity panel in the United States has already taken the unusual step of asking the scientists to keep part of their data secret to prevent others from reproducing their work. The experiments involve a type of bird flu virus known as H5N1, which rarely infects people but is highly deadly when it does. Since 1997, when the virus was first identified, about 600 people have been infected, and more than half died — an extraordinarily high death rate. The saving grace of H5N1 is that when people do become infected — nearly always from contact with birds — they almost never transmit the disease to other people. But the virus has persisted in the environment, infecting millions of birds, and scientists have warned that if it mutates to become more contagious in people, disaster could ensue. But what mutations would make the virus more easily transmissible? And how hard, or easy, would it be for those mutations to occur? Hoping to answer those questions, some researchers began experimenting with bird flu, working with ferrets, which are considered the best model for studying flu, because they contract it and get sick in much the same way that people do. Recently, two research teams — one at Erasmus Medical Center in Rotterdam, in the Netherlands, and the other at the University of Wisconsin, Madison — announced that they had produced a form of H5N1 with mutations that allowed it to “go airborne,” meaning that it spread through the air from one ferret to another. Presumably, though not certainly, the virus could spread in the same way among people. Ron Fouchier, a virologist who conducted the research in Rotterdam, said he was surprised by how easy it was to change the virus into the very form that the world has been dreading. Now, the world must decide what to do with it.